The Supreme Court’s ruling last week, allowing police to compel DNA samples from persons arrested for serious offenses, will solve cold cases around the country and put dangerous criminals behind bars. But despite this clearly beneficial impact, the court’s 5 to 4 ruling was wrong — and may be more far-reaching than we can now imagine.

The words “Antonin Scalia was right” do not flow easily for me. But the court’s most uncompromising conservative, who wrote a withering dissent, was correct when he issued a dire-sounding warning from the bench: “Make no mistake about it: As an entirely predictable consequence of today’s decision, your DNA can be taken and entered into a national database if you are ever arrested, rightly or wrongly, and for whatever reason.”

The case, involving a Maryland law that mandates DNA collection, scrambled the court’s ideological seating chart. Scalia, of all people, sided with the liberals, while Justice Stephen Breyer, a liberal, joined the conservative majority.

Maryland v. King was an appropriate test case. A man named Alonzo King was arrested in 2009 on an assault charge. Police in Wicomico County took a DNA sample by swabbing the inside of his cheek — without obtaining a search warrant — as permitted under Maryland law. Months later, King’s DNA profile was matched with evidence from a 2003 rape case. He was subsequently tried and convicted of the rape.

It’s impossible not to applaud the result: A rapist who otherwise would have escaped justice was made to pay for his heinous crime. But the way this result was obtained, Scalia argued, ignores the Constitution.

Writing for the majority, Justice Anthony Kennedy accepted the state’s argument that the DNA sample was actually a method of identification — like a mug shot or a set of fingerprints — and not an unreasonable search.

With all due respect, this is a bunch of hooey. As Scalia put it, Kennedy’s argument “taxes the credulity of the credulous.”

Before the DNA test was even performed, police knew perfectly well who King was. They had his name, address, date of birth, height, weight, eye color, you name it. No question had been raised about his identity.

Months elapsed before King’s DNA was entered into a national database. If identity were the purpose for collecting the sample, you’d think it would be compared with the DNA of people who looked like King or had a similar name. Instead, it was compared with DNA samples collected at the scenes of unsolved crimes.

In other words, the obvious purpose of collecting the DNA sample was to solve cold cases. This is an admirable goal. But there’s that pesky Fourth Amendment.

“Solving unsolved crimes is a noble objective,” Scalia wrote, “but it occupies a lower place in the American pantheon of noble objectives than the protection of our people from suspicionless law-enforcement searches.”

I don’t pretend this is an easy case. My first reaction would be to try to find some way to uphold King’s conviction — and give police a way to catch others who believe they’ve gotten away with crimes such as rape or murder. But the court’s solution is based on the kind of jury-rigged legal reasoning that can end up creating more problems than it solves.

The ruling upholds laws already in place in 28 states; I imagine most or all of the rest will soon pass DNA laws of their own. If DNA collection is allowed for suspects arrested for alleged felonies, why not misdemeanors? Why not, for that matter, whenever police stop someone whose identity is deemed questionable because of, say, an expired driver’s license?

I realize that the law must come to terms with DNA and its unique power as evidence. I realize this will be an awkward and uncomfortable process. But we should move forward with both the letter and the spirit of the Constitution in mind. As Scalia noted, the Founders “would not have been so eager to open their mouths for royal inspection.”

In that spirit, before we give police such a broad right to compel DNA testing to put individuals in prison, we should at least guarantee that individuals who are incarcerated have a similar right to DNA testing that might exonerate them. And we should be honest about why we’re making people say “Aah.”

Eugene Robinson writes a twice-a-week column on politics and culture, contributes to the PostPartisan blog, and hosts a weekly online chat with readers. In a three-decade career at The Post, Robinson has been city hall reporter, city editor, foreign correspondent in Buenos Aires and London, foreign editor, and assistant managing editor in charge of the paper’s Style section.

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